The present invention relates to a catalyst for the synthesis of methyl mercaptan from methanol and hydrogen sulfide, and a process for preparing this catalyst. In a further aspect, the present invention relates to a process for the synthesis of methyl mercaptan utilizing the aforesaid catalyst.
Methyl mercaptan is an important industrial intermediate for the synthesis of methionine and for production of dimethyl sulfoxide and dimethyl sulfone. At present it is produced predominantly from methanol and hydrogen sulfide by reaction in the presence of an aluminum oxide catalyst. Methyl mercaptan is usually synthesized in the gas phase at temperatures from 300.degree. C. to 500.degree. C. and at pressures of 1 to 25 bar.
The reaction gas mixture contains not only the methyl mercaptan produced but also the unreacted starting materials and byproducts such as dimethyl sulfide and dimethyl ether, as well as gases which are inert in this reaction, such as methane, carbon monoxide, hydrogen and nitrogen. The methyl mercaptan produced is separated from this reaction mixture.
If the reaction of hydrogen sulfide and methanol is carried out in the presence of the catalyst at elevated pressure so that the product of methyl mercaptan preparation is at elevated pressure (more than 7 bar), methyl mercaptan can, for example, be separated by washing with methanol at a washer head temperature of 25.degree. C., as described in German patent 17 68 826. If the reaction product is at normal pressure, it is necessary to recover the product at temperatures down to -60.degree. C. (Japanese Laid-Open Patent Application 45-10728) to obtain the methyl mercaptan in liquid form. The unreacted hydrogen sulfide can be returned to the reactor, as described in German Patent 17 68 826.
The highest possible selectivity is required in the catalytic reaction of methanol and hydrogen sulfide to produce methyl mercaptan, so as to keep the cost of separating the methyl mercaptan produced as low as possible, for the process to be economically feasible. Here the cost of energy for cooling the reaction gas mixture to condense the methyl mercaptan is a major cost factor.
The aluminum oxide catalyst is usually activated with potassium tungstate to increase its activity and selectivity. The activator is usually used in proportions of up to 15% of the total weight of the catalyst. The activity and selectivity can also be improved by increasing the molar ratio of hydrogen sulfide to methanol. Usually molar ratios of 1 to 10 are used.
A high molar ratio, of course, means that the hydrogen sulfide is in great excess in the reaction gas mixture, so that large volumes of gas must be recirculated. The ratio of hydrogen sulfide to methanol should be only slightly different from 1 to reduce the cost of energy for recirculation. It is also desirable to carry out the reaction at the lowest possible temperature to reduce heat loss from the reactor.
U.S. Pat. No. 2,820,062 describes a process for producing organic thiols using an activated aluminum oxide catalyst which has been activated with potassium tungstate in proportions of 1.5% to 15% by weight, based on the weight of the catalyst. This catalyst gives good activities and selectivities at a reaction temperature of 400.degree. C. and a molar ratio of 2. This U.S. patent mentions various possibilities for incorporating the potassium tungstate into the aluminum oxide, including impregnation processes, coprecipitations, and simple mixtures. The actual catalyst production is considered of little importance to the economics of the process for methyl mercaptan synthesis.
An object of this invention is to improve the methyl mercaptan synthesis in terms of improved activity and selectivity at low molar ratios of hydrogen sulfide to methanol, in comparison with the known methods using known catalysts, and which therefore gives a more economical process.